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As I understand, a basic tile server provides raster image tiles for different zoom levels. To cover the world, it requires 1 tile at zoom level 0, 4 at zoom level 1, 4n for zoom level n. These are billions and billions of images that need to be stored in total for zoom levels 0 to 19.

My question is: Do tile servers typically in fact store all of these images? If not, how do they cover the whole world at every zoom level with tiles?

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No, they don't store every tile. There are several reasons for this:

  • Size: Storing all tiles for all zoom levels would require more than 50 TB storage (source: Tile Disk Usage)
  • Gain: Typically, only 1.79% of all tiles are actually viewed since many just contain water or other uninteresting regions (source: Tile Disk Usage)
  • Computational costs: Pre-rendering all tiles would take a really long time.

Typically, tile servers pre-render some low-zoom tiles (let's say zoom levels 0-12) because they take long to render and will be viewed by many users. For higher zoom levels tiles are rendered on the fly and then cached for a certain time.

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